Effect of fruit load on the photosynthetic capacity and on the ecological light use efficiency of an apple orchard

In many horticultural crops, fruit load is known to have a direct effect on photosynthesis and dry matter production. In this study we analyzed the effect of the different fruit load occurred in five successive growing seasons, on the photosynthetic capacity (F_m) and the ecological light use efficiency (Ɛ_ecol) of an apple orchard where photosynthesis (GPP) was continuously derived from eddy covariance measurements of net carbon exchange. Both F_m and Ɛ_ecol were statistically different between high (HFL) and low (LFL) fruit load seasons in September and when considering August, September and October as a single group (p-value<0.05). A significant direct linear regression (slope = 0.186±0.034) was found between the average F_m of these three months and the amount of fruit production (t ha^-1) recorded every year. Besides the limited number of years of measurements, this study provided evidence of a direct linear effect of the fruit load on the photosynthetic capacity and the ecological light use efficiency of the apple orchard at the ecosystem level, supporting the sink limitation theory.